1. Field of the Invention
The invention relates to an air-fuel ratio imbalance detecting device and air-fuel ratio imbalance detecting method for an internal combustion engine.
2. Description of Related Art
There is a device in which, in a state where an operating state of an internal combustion engine is a predetermined steady operation state, when a variation per unit time in air-fuel ratio detected by an air-fuel ratio sensor provided in an exhaust pipe falls outside a predetermined range, it is determined that there is an imbalance in air-fuel ratio among cylinders of the internal combustion engine (for example, see Japanese Patent Application Publication No. 2011-144785 (JP 2011-144785 A)). In this way, the device described in JP 2011-144785 A simply determines whether there is an imbalance in air-fuel ratio among the cylinders of the internal combustion engine.
In the above-described air-fuel ratio imbalance detecting device for an internal combustion engine, it is possible to detect an imbalance in air-fuel ratio among the cylinders of the internal combustion engine by normalizing a variation per unit time in the air-fuel ratio of the internal combustion engine with a constant based on an intake air amount and then comparing the normalized variation per unit time in the air-fuel ratio with a predetermined threshold.
However, a normalized value obtained by normalizing a variation per unit time in the air-fuel ratio of the internal combustion engine with a constant based on an intake air amount does not incorporate a difference in load factor at the same intake air amount, so a difference between a normalized value in the case where there is an imbalance in air-fuel ratio among the cylinders of the internal combustion engine and a normalized value in the case where there is no imbalance may be small due to a difference in load factor.
Thus, the above-described air-fuel ratio imbalance detecting device for an internal combustion engine may not be able to detect an imbalance in air-fuel ratio among the cylinders of the internal combustion engine or may erroneously detect an imbalance.